This invention relates to superalloy compositions particularly suited for producing parts to be employed for high temperature applications. The compositions can be effectively produced utilizing single crystal casting techniques and can also be utilized for the production of polycrystalline components such as directionally solidified, conventionally solidified (equiaxed), wrought or dispersion strengthened materials.
It is well known that metal articles are exposed to extreme temperatures in certain applications such as in gas turbine engines. When used for such applications, the articles must have good strength and ductility at the high temperatures, and they must exhibit suitable surface stability in view of the corrosive conditions encountered. Desired characteristics also include metallurgical stability and ease of casting.
Alloy compositions typically used for such applications are not particularly susceptible to the addition of elements which are beneficial to surface stability. Such elements may have a tendency to weaken grain boundaries and, therefore, the improvements in surface stability result in a loss of strength at elevated temperatures. Where elements such as boron, carbon or hafnium are introduced to modify grain boundaries, the elements can be detrimental to surface stability and, therefore, a loss of some properties to achieve an improvement in others is also realized in those cases. Finally, random orientation of grain boundaries in prior alloys has been recognized, and this results in grain boundary "slide" and "creep" at elevated temperatures.
The conventional casting alloys referred to which will achieve suitable performance at a certain temperature limit include, for example, an alloy of the following composition designated B-1900+Hf; Cr-8%; Co-10%, Al-6%, Ti-1%, Ta-4.3%, Cb&lt;0.1%, Hf-1.1%, C-0.1%, Zr&lt;0.13%; B-0.015%, Fe&lt;0.35%, Mn&lt;0.2%, Si&lt;0.2%, Mo-6%, Ni-balance.
The grain boundary sliding phenomenon usually occurs at the "transverse" grain boundaries (boundaries transverse to the stress direction) and can be eliminated by removing these boundaries in the alloy. Such a casting approach is called directional solidification (D.S.) wherein grains are made to grow in a preferred orientation parallel to the direction of heat flow. Although these alloys possess better creep strength and thermal shock resistance than the conventionally cast alloys, they still have grain boundaries which control the type of alloying elements that can be added to enhance the useful life of the alloy. An example of a nickel-base D.S. alloy is Mar-M200+Hf which has the following composition: Cr-9%, Co-10%, Al-5%, Ti-2%, Cb-1%, W-12%, Hf-2%, C-0.14%, Zr-0.05%, B-0.02%, Ni-balance. It is seen that most of the grain boundary modifiers are still needed for this alloy.
The above mentioned grain boundary modifiers can be removed using a single crystal casting approach in which there are no grain boundaries. Alloys proposed for this approach generally possess superior creep resistance and high temperature strength when compared with conventionally cast or directionally solidified alloys. This beneficial attribute makes it desirable to use these alloys for very high temperature (2000.degree. F.) applications. There is, however, still that problem of surface stability at these high temperatures, the primary modes of surface attack being oxidation and thermal fatigue and to a lesser extent, sulfidation. An example of a single crystal alloy, currently in use, is Monoloy 454 with the following composition: Cr-10%, Co-5%, Al-5%, Ti-1.5%, Ta-12%, W-4%, and Ni-balance.